JP5053531B2 - Fresnel lens - Google Patents

Description

  The present invention relates to a Fresnel lens.

  The Fresnel lens is a lightweight lens with a minimum thickness required to achieve tilting by replacing the inclined surface of a convex or concave lens with a discontinuous inclined surface with multiple concentric or parallel prisms. It is a compact flat lens.

  A Fresnel lens, for example, a lens used for a backlight of a liquid crystal display device (for example, see Patent Document 2 below) is used to convert light from a point light source into parallel rays, and on the contrary, in a solar power generation device. Like a condensing lens (for example, the following patent document 3), it is widely used for condensing parallel rays.

  In a Fresnel lens having a small F value (focal length / lens diameter), the angle (deviation angle) at which light is bent at the periphery of the lens far from the optical axis is large. In such an area, the slope of the prism becomes large and the incident angle to the refractive interface increases, so that the ratio of reflection at the refractive interface increases and the transmittance decreases. Therefore, in areas where a large declination is required, it is known that the transmittance can be improved by designing the vertex angle of the prism so that it is refracted on the vertical surface after being totally reflected by the slope of the prism. For example, Patent Documents 1, 2, and 4 listed below also describe such Fresnel lenses.

US Pat. No. 4,755,921 JP 2002-221605 A JP 2004-172255 A US Pat. No. 4,337,759

  The present invention provides a Fresnel lens using a prism with improved transmittance compared to the prior art.

  According to the present invention, there is provided a Fresnel lens including a plurality of prisms, and at least a part of the plurality of prisms at least receives incident light from a focal point or incident light parallel to the optical axis from the opposite side. A Fresnel lens having an apex angle that is totally reflected twice is provided.

  The Fresnel lens includes, for example, a first area composed of a prism having an apex angle that does not totally reflect the incident light, and a prism having an apex angle adjacent to the first area that totally reflects the incident light twice. It has two areas.

  The Fresnel lens may further include a third section that is composed of a prism adjacent to the second section and having an apex angle that totally reflects the incident light once inside.

By introducing a prism that totally reflects twice or more inside, reflection on the refracting surface is reduced, and the transmittance can be further improved.
[Embodiment of the Invention]

  Only the direction of light is different between a condensing Fresnel lens used in a solar power generation device or the like and a Fresnel lens for obtaining a parallel light beam used in a backlight of a liquid crystal display device or the like. Therefore, although the following description will be made on a condensing Fresnel lens, this description can be similarly applied to a Fresnel lens for obtaining parallel rays. Although a circular Fresnel lens in which prisms are arranged concentrically will be described, the present invention can be similarly applied to a linear Fresnel lens in which prisms are arranged in parallel.

  The condensing Fresnel lens has a type in which light is incident from a Groove surface 12 in which a large number of grooves 10 for forming a large number of prisms are formed as shown in FIG. 1, and a flat surface 14 as shown in FIG. There is a type that enters from the side. In the type incident from the Groove surface 12, as shown in FIG. 1, the light 16 incident on the portion close to the vertical surface 17 of the prism is totally reflected by the vertical surface 17 and is not finally collected at the target focal position. Does not contribute to the light collection rate. For this reason, in order to condense sunlight with high efficiency, the type of incident light from the plane as shown in FIG. However, as the distance from the optical axis increases, the slope 18 of the prism becomes steeper, and as a result, the ratio of reflection occurring at the interface increases, leading to a reduction in the light collection rate. Therefore, the part away from the optical axis has been designed using a prism of the type that is totally reflected by the inclined surface 18 and refracted by the vertical surface 17 as shown in FIG. Thereby, a high condensing rate can be obtained. In particular, in a region where the F value (focal length / lens diameter) represented by the ratio of the focal length and the lens diameter is small, a light collection rate that is particularly effective and superior to a refractive Fresnel lens can be obtained.

  By the way, in the total reflection type prism that has been used so far, as shown in FIG. 4, light incident perpendicularly to the plane a undergoes total reflection once on the inclined surface c and refracts on the vertical surface b. To focus.

  On the other hand, in the prism as shown in FIG. 5, the light incident perpendicularly to the plane a undergoes total reflection at the inclined surface c, and further causes the second total reflection at the vertical surface b, and is refracted at the inclined surface c. To go to the focus. In such a prism, in order to obtain the same declination, the incident angle at the time of refraction is reduced, and the loss due to Fresnel reflection is reduced. As a result, compared with the conventional catadioptric Fresnel lens having a prism that causes total internal reflection once, the catadioptric Fresnel lens having a prism that causes total internal reflection twice or more has a higher light collection efficiency. Is expected to be obtained.

In the case of the refraction type shown in FIG. 6, the declination β is β = sin ⁻¹ (nsin α) −α where α is the apex angle of the prism and n is the refractive index of the prism material.
It becomes.

The declination β (deg) in the case of one-time total reflection shown in FIG.
β = 90−sin ⁻¹ (nsin (2α−90))
And
The declination β in the case of two-time total reflection shown in FIG.
β = α-sin ⁻¹ (3α−180)
It becomes.

In general, when m is an integer, and m is an even number and m total reflections occur internally, the declination β is
β = α−sin ⁻¹ (nsin ((m + 1) α−90 m))
And
If m is odd,
β = 90−sin ⁻¹ (nsin (m + 1) α−90m))
It becomes.

  In the case of acrylic resin having a refractive index of 1.49 in Tables 1 to 5, for prisms of refraction type, once total reflection type, two times total reflection type, three times total reflection type, and four times total reflection type, The calculation results of the deflection angle β and transmittance I with respect to the apex angle are shown. The range marked with “*” in the table indicates the range in which the transmittance decreases due to interference with adjacent prisms. FIG. 7 shows a graph of these.

  When Table 1 and Table 2 are compared, as is clear from the prior art, in the range where the deflection angle β is larger, the one-time total reflection prism can obtain a higher transmittance than the refractive prism. Recognize. For example, when the apex angle α is 38.5 ° with a refractive prism, the declination β is 30.0 °, and the transmittance I at this time is 0.813, whereas the apex angle that gives the same declination In the one-time total reflection type in which α is 62.7 °, the transmittance I is 0.875, which is higher than that.

  Further, referring to Table 3, it is noted that even in the two-time total reflection type, when the apex angle α is 68 °, the deflection angle β = 30 ° is obtained, and the transmittance I at this time is 0.914, which is even higher.

  FIG. 8 plots the transmissivity I on the horizontal axis and transmissivity I on the vertical axis, and plots the transmissivity I of each type of prism against the declination β. As is clear from FIG. 8, in the case of an acrylic resin having a refractive index of 1.49, the refractive prism has the highest transmittance I in the region 20 where β <19.5 °, and 19.5 ° <β <31.0. It can be seen that the two-time total reflection type prism has the highest transmittance I in the region 22 at °, and the one-time total reflection type prism has the highest transmittance I in the region 24 at 31.0 ° <β.

  Table 6 shows lenses having various F-numbers when only a refraction prism is used (referred to as Prior Art 1) and a total reflection prism is used once at 20 ° ≦ β (referred to as Prior Art 2). ), And according to the present invention, the light condensing rate of 20 ° ≦ β ≦ 29 ° using a total reflection prism twice and 29 ° ≦ β using a total reflection prism once (denoted as the present invention) is shown. FIG. 9 shows a graph of this.

  As is apparent from Table 6 and FIG. 9, it can be seen that the lens having the F value of 1.00 or less has a high light collection rate when the Fresnel lens of the present invention using a partially reflecting prism in part is high.

  A case of making a square circular Fresnel lens (concentric Fresnel lens cut into a square) having a 200 mm square and a focal length of 200 mm using an acrylic resin having a refractive index of 1.49 will be described. The 200 mm square lens has a diagonal of 282 mm, and the F value is 0.709 on a square diagonal, which is 1 or less. Therefore, the light collection efficiency of the entire lens is considerably lowered with a refraction-only prism. Therefore, a total reflection prism is introduced in a portion away from the optical axis. A total reflection prism that causes two total internal reflections at a position where the light transmittances of the refraction prism and the reflection prism are reversed are arranged as shown in FIG. In FIG. 10, a prism having a refractive index is used when the declination β is 0 ° to 19.5 °, and a prism that causes two total reflections is arranged from 19.5 ° to 31 °. One total reflection prism is arranged up to 26 degrees. The Fresnel lens thus made has better light collection efficiency than a refractive prism and a single total reflection prism.

  Tables 7 to 9 show the calculation results of the deflection angle β and the transmittance I of the refraction type, the once total reflection type, and the twice total reflection type in the case of silicon rubber having a refractive index n = 1.40. FIG. 11 shows the transmittance I plotted against the deflection angle β for each type of prism.

  From FIG. 11, in the case of silicon rubber with n = 1.40, as shown in FIG. 12, a refractive prism is used in the region 30 where β <18.2 °, and 18.2 ° <β <31.8 °. If the total reflection type prism is used twice in the area 32 and the total reflection type prism is used once in the area 34 where 31.8 ° <β, a Fresnel lens having a high light collection rate can be obtained.

It is a figure which shows the Fresnel lens for condensing in the case of injecting from the groove surface side. It is a figure which shows the Fresnel lens for condensing in the case of entering from a plane side. It is a figure which shows the Fresnel lens for condensing which uses a 1 time total reflection type prism for a part. It is a figure explaining the optical path of a 1 time total reflection type prism. It is a figure explaining the optical path of a 2 times total reflection type prism. It is a figure explaining the optical path of a refraction type prism. 6 is a graph in which the deflection angle β and the transmittance I are plotted against the apex angle α. It is the graph which plotted the transmittance | permeability I of each type of prism with respect to the deflection angle (beta). It is the graph which compared the condensing rate of the Fresnel lens of this invention, and the Fresnel lens of a prior art. It is a figure which shows an example of the Fresnel lens of this invention at the time of using the acrylic resin of refractive index 1.49. It is the graph which plotted the transmittance | permeability I of each type of prism at the time of using the silicon rubber of refractive index 1.40 with respect to the declination (beta). It is a figure which shows an example of the Fresnel lens of this invention at the time of using the silicon rubber of refractive index 1.40.

Claims (4)

A Fresnel lens comprising a plurality of prisms, the first prism comprising an apex angle that is located at the center of the lens and does not totally reflect incident light from a focal point or incident light parallel to the optical axis from the opposite side . A Fresnel lens comprising a zone and a second zone consisting of a prism located outside and having an apex angle that totally reflects the incident light twice inside the prism . The second and further located outside the zone, the third Fresnel lens of claim 1, further comprising an area consisting of a prism having an apex angle which once totally reflected incident light inside the prism. It is made of a material having a refractive index of 1.485 or more and less than 1.495, the deflection angle of the prism belonging to the first section is 20 degrees or less, and the deflection angle of the prism belonging to the second section is 19 degrees or more and 32 degrees. The Fresnel lens according to claim 2 , wherein the deflection angle of the prism belonging to the third area is 30 degrees or more. A prism made of a material having a refractive index of 1.395 or more and less than 1.405, the deflection angle of the prism belonging to the first section is 19 degrees or less, and the deflection angle of the prism belonging to the second section is 18 degrees or more and 32 degrees. 3. The Fresnel lens according to claim 2 , wherein a deflection angle of the prism belonging to the third section is 31 degrees or more.

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